The invention relates to a slider bearing for use with an apparatus comprising a vacuum chamber, the slider bearing comprising:                a base plate in contact with the vacuum chamber at one side, said base plate showing a first through-hole in contact with the vacuum chamber,        a second plate, one side of the second plate in contact with the base plate, said second plate showing a second through-hole,the faces of the base plate and the second plate facing each other being sufficiently smooth to form a non-elastomeric vacuum seal,said base plate and said second plate slidable between a first relative position in which the first through-hole and the second through-hole do not overlap and a second relative position in which the first through-hole and the second through-hole overlap. Such a slider bearing is known from European application No. 05076474, published as EP1 622 185 A1.        
Such a slider bearing is used in e.g. a tabletop Scanning Electron Microscopes (tabletop SEM). A tabletop SEM is a SEM which is both much smaller and much cheaper than conventional SEM's. Such tabletop SEM's are commercially available from e.g. FEI Company under the name Phenom.
The known slider bearing comprises a base plate on which an electron-optical column is mounted. The electron-optical column produces a focused beam of electrons along an electron-optical axis. The base plate shows a through-hole in contact with the evacuated inner volume of the electron-optical column, centred round the electron-optical axis. The base plate is placed against a second plate in such a way that the first and the second plate may slide over each other and that a vacuum seal is formed between the two plates, thereby sealing the evacuated inner volume of the electron-optical column. The second plate shows a depression in which a sample is placed.
To insert the sample in the depression the plates are positioned such that the through-hole in the base plate is covered by the second plate (thus sealing the evacuated inner volume of the electron-optical column), and the depression is open to atmosphere (thus enabling entrance from outside). To observe the sample the depression is aligned with the through-hole in the base plate by sliding the two plates over each other. Sliding the two plates over each other also performs fine alignment of an area of interest on the sample with respect to the electron-optical axis.
As known to the person skilled in the art vibrations are a major limitation for the resolution obtained with particle-optical instruments. The known slider bearing uses a metal-to-metal seal, without using an elastomer in the form of e.g. an O-ring. An advantage of a slidable seal not using elastomers is that it results in a very stiff coupling of the electron-optical column to the sample, and thus a low sensitivity to vibration. Therefore a non-elastomeric seals is preferred over the more commonly used elastomer seals, such as O-ring seals.
In the known slider bearing the force with which the two plates are pressed together depends on the area enclosed by the contour of the vacuum seal. At the interface between the base plate and the second plate the area within the contour can be thought to be evacuated, the area outside the contour to be connected to atmosphere. The force with which the two plates are pressed together is thus the evacuated area enclosed by the contour multiplied with the atmospheric pressure. To slide the two plates over each other, the (static) friction force between the two plates must be overcome, said static friction force dependent on the force with which the two plates are pressed together.
A disadvantage of the known slider bearing is that the contour where the vacuum seal is formed is not well defined: e.g. a slight curvature or unevenness of one of the two plates may change the contour where the actual seal forms. As a result the force with which the two plates are pressed together is likely to vary with the position of the plates relative to each other. This in turn results in a varying frictional force between the two plates when sliding them over each other, and thus a different loading of the actuators that slide the two plates over each other. This change in load of the drive is contrary to the demands of a high precision and/or low backlash drive. It also necessitates the use of a drive that is more powerful than needed, resulting in a larger and a more expensive drive for the slider bearing.
Another disadvantage of the known slider bearing is that during sliding particles may be produced at those places where the pressure occurring locally is too high. These particles can be introduced in the electron-optical column of the tabletop SEM, where they can give rise to e.g. charging. They can also be introduced on the sample, and be mistaken for parts of the sample, thereby giving false information about the sample.
The invention aims to provide a slider bearing that does not show these disadvantages.